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Feature

Astronomers in England have discovered a
singing black hole in a
distant cluster of galaxies. In the process
of listening in, the team of
astronomers not only heard the
lowest sound waves from an object in
the Universe ever detected by humans, but
they've also discovered an
important clue about the formation of galaxy
clusters -- the largest
structures in the cosmos.

Dr. Andrew Fabian and his colleagues at
the Institute of Astronomy in
Cambridge, England made their discovery
using NASA's Chandra X-ray
Observatory, an orbiting X-ray telescope
that sees the Universe in X-ray
light just as the Hubble Space Telescope
sees it in visible light.

The black hole is situated in the center
of a galaxy amid a group of
thousands of galaxies collectively called
the Perseus Cluster and
located 250 million light years from Earth
(meaning it took the light
from these galaxies 250 million years
to reach us). The sound
waves coming from it are in the form of
a single note, so rather than a
song it is really a drone.

Sound waves in the Perseus Cluster

Using the piano keyboard's middle C note
as a reference point for the
middle of the piano key music range, Fabian's
team determined the note
is a B -flat. On a piano, the B-flat nearest
middle C is located midway
between 1/8th and 2/8th of an octave away.
In musical terminology, this
B flat is 1-1/2 steps from middle C.

The Perseus cluster black hole's B-flat,
by contrast, is 57 octaves
below middle C or one million, billion times
lower than the lowest sound
audible to the human ear! In terms of frequency
(the time it takes a
single sound wave to pass by), the lowest
sounds a person can hear is
1/20th of a second. The Perseus black hole's
sound waves have a
frequency of 10 million years!

You may be wondering how a sound wave can
travel through space. After
all, sound waves require some sort of stuff
to move through. This stuff,
called a medium, can be air, water, or even
solid rock. And space is
thought of as lacking any medium because
it is a vacuum.

In fact, space is not a pure vacuum but
rather it contains stray bits of
stuff -- gas atoms and dust of varying amounts.
In the case of the
Perseus cluster, the gas throughout it serves
as the medium through
which the sound waves coming from the central
black hole travel.

Still image from animation
of sound waves generated in perseus cluster

The sound waves were indirectly detected
using the Chandra telescope
because the cluster gas is very hot and
thus emits an especially
energetic form of light called X rays, as
well as less energetic visible
light. And the gas is so hot because of
the effects of the black hole.

More than an acoustic curiosity, these sound waves
transport energy that keeps gas throughout the cluster
warmer than it would otherwise be. These warmer temperatures,
in turn, regulate the rate of new star formation,
and hence the evolution of galaxies and galaxy clusters.
This makes the findings far more significant for understanding
the astrophysical evolution of the Universe.

The Perseus sound waves are much more
than just an interesting form of
black hole acoustics, said Steve Allen,
also of the Institute of
Astronomy and co-investigator in the study.
These sound waves may be
the key in figuring out how galaxy clusters,
the largest structures in
the Universe, grow.

Astronomers will now analyze other galaxy
clusters for similar sound waves.